Our invention relates to a method of reducing the NO.sub.x -content in flue gas generated on firing a rich gas coke oven or combination coke oven and a coke oven battery for performing that method.
The process of our invention may be applied to a rich gas coke oven or a combination coke oven with heated walls forming dual heating flues, with a single stage or multistage combustion in the dual heating flues and with regeneration for recovery of flue gas heat and preheating of the underfiring media.
The NO.sub.x -formation depends among other things on the flame temperature, which is naturally quite high(up to about 3000.degree. C.) because of the high air preheating and because of the high heat content with rich gas during coke oven firing. In coke oven firing with lean gas, because of the lower gas heat content and the greater flue gas generation, lower flame temperatures and thus reduced NO.sub.x concentrations result in the flue gas than with rich gas.
Internal flue gas recirculation has been used for reduction of the NO.sub.x -content in the flue gas in a coke oven with dual heating flues with an upper flue gas circulation turning point and lower connecting opening in a common central wall between the heating flues. In this kind of coke oven the flue gas, which is automatically recirculated inside the dual heating flues, is transported from the downwardly-directed heating flues into the rising heating flues and/or the combustion containing heating flues.
External flue gas feed back, in which flue gas drawn out of the flue is intermixed with the hot gas and/or the combustion air prior to or on admission to the oven, has also been known and utilized.
Both known methods using internal and external flue gas recirculation up to now have been only used independently of each other.
External flue gas recirculation requires several additional measures: Blowers for the flue gas and if necessary also air return ducts, a feed and distribution system with control and regulatory devices, greater regenerators, increased underfiring and additional flow requirements. Because of the increased investment and operating costs the external flue gas recirculation has almost no practical importance. The internal recirculation method in contrast is not burdened with its disadvantages and thus finds greater use. Both processes have specific limitations in regard to their attainable feed back rate. However external flue gas recirculation has a greater cooling action than internal flue gas recirculation, when the two methods are compared at the same flue gas flow rates, because the admission temperature into the fired heating flues is somewhat lower in the case of external flue gas recirculation.